The purpose of the proposed research is to study the regulation of active phosphate reabsorption by the isolated urinary bladder of the toad, Bufo marinus. The principal investigator has recently demonstrated that the toad bladder actively transports phosphate and that this transport is inhibited by dibutyryl cyclic AMP, an analogue of cyclic AMP, the intracellular mediator of parathyroid hormone. The problem will be investigated further by studying bidirectional fluxes of labelled phosphate transport in the toad bladder. A number of factors which may regulate phosphate transport in the toad bladder will be tested. These include parathyroid hormone, theophylline, phosphate, calcium, sodium and hydrogen ion concentrations in the bathing media, variation in the energy source supplied to the bladder and vitamin D. In addition, the effects of metabolic inhibitors and ouabain will be tested. Finally, the intracellular concentration of inorganic phosphate will be measured using isolated toad bladder epithelial cells. This will allow a localization of the site of active transport of phosphate at either the mucosal or serosal border of the cell. The toad bladder has been a useful model in the study of renal mechanisms of water and sodium reabsorption, as well as the actions of vasopressin and aldosterone on the kidney. It is anticipated that the proposed studies will provide important information regarding the regulation of phosphate excretion by the kidney.